This invention relates to natural draught cooling towers and to a method and apparatus for preventing cold air break-ins at low wind velocities and the formation of a vortex over the tower at high wind velocities.
Natural draught cooling towers are well known. The purpose of a natural draught cooling tower is to extract the heat from the heated coolant water of a thermal power station, a manufacturing process, or the like. The coolant water gives off its heat to the ambient air which is conveyed upwardly in the cooling tower by the natural uplift of the ambient air being heated in the cooling tower.
As is well known, the cooling tower separates the relatively warmer air within the tower from the relatively cooler air outside the tower. As the heated air rises within the tower, the heavier cooler air is pulled into the tower at the lower end for warming. The tower must, of course, have a side wall which is closed, i.e. without apertures, to maintain the separation of the two air masses. The difference in the temperature of the two air masses is reflected in their pressure and the pressure differential between the air inside and outside of the cooling tower is a maximum at the bottom of the tower and decreases as a function of the height of the tower to the crown where the pressures are the same.
Prior to the present invention, the crown portion of cooling towers was enlarged to have effect as a diffusor and thus to increase the effectiveness of the cooling tower by a partial regeneration of pressure energy. The hyperbolic shape has become the standard. The universally adopted rule has been to avoid acceleration of the plume beyond that necessary to generate the required uplift. See, for example, the article "Gegenwartige Kuhlturmtechnik" ("Cooling Tower Techniques of Today") by Dr. Ing. Paul Berliner, Karlsruhe in the Journal "Warme" (Heat) pp. 25-29, Vol. 80, 1974.
The phenomenon of cold air penetration downwardly into the top of the cooling tower in still air generally has not been considered a problem. However, recent studies of the inventors have shown that cold ambient air in still air conditions flows into the tower to form a ring inside the crown of the cooling tower. Since the pressure inside and outside the tower is thus equalized to the extent of the cold air penetration, the crown position of the tower is not effective and the effective height of the tower is decreased. The plume is accelerated and the air flow from the tower is diminished.
These recent investigations (published in Fortschrittsberichte V.D.I.-Z., Series 15, No. 5, July, 1974) have shown that the weather conditions can substantially influence the functioning of the cooling tower. As discussed above, the known cylindrical and hyperbolic forms of cooling tower promote, in low wind velocities, the pentration of colder and therefore heavier air into the outlet opening at the top of the cooling tower. As a result, the effective height of the cooling tower can be reduced by up to 25% and more. A second and related problem exists with high wind velocities, where the wind produces a dead region in the form of a flow vortex in the cooling tower outlet. This vortex partially obstructs the cooling tower outlet and, with a wind velocity of 20 m/s, can reduce the effective uplift height of the cooling tower as much as 30%.
The present invention has as a principal object the development of a novel cooling tower in which the effects of weather conditions on the performance of the tower are significantly reduced.
In regard to cold air penetration, one feature of the invention is to provide a cooling tower having a crown tapered inwardly towards the upper opening rim. In this connection, the term "crown" is used to mean the upper end portion of the cooling tower wall, having an axial length which is small in comparison with the total height of the tower.
The new cooling tower of the present invention is designed so that the pressure gradients in the tapered crown region of the cooling tower show the following relative behavior inside and outside the cooling tower in still air and wind velocities less than about 10 m/s: ##EQU2## in which p=pressure, i=inside the tower, a=outside and z=the vertical height coordinates measured downwardly from the upper rim of the crown. In the tapered crown region, there is produced a barrier layer, which prevents the penetration of cold air, because the sum of the specific gravity of the heated plume and volume-related inertia forces is greater than the specific gravity of the cold outside air.
It is an important advantage that the tapered region, initiated with a bend or angle, stiffens the casing of the cooling tower, so that it is possible to dispense with the usual stiffening or reinforcing ring which surrounds the crown of conventional cooling towers.
As regards the design in practice, there are also to be taken into account the different temperatures, gas constants and densities of the media inside and outside the cooling tower. The tower itself must have a height of at least 80 m. to provide the necessary updraught and the interior thereof should be free of corners. The height H (axial length) of the tapered crown region is determined, in practice, to be between 3% and 10% of the total height of the cooling tower, preferably 5%. The ratio between height H of the cooling tower crown and the largest diameter D of the crown, i.e., the diameter at the lower end thereof, may be between about 1 to 12 and about 1 to 3.
One suitable height-diameter ratio H/D is in the order of magnitude of 1 to 7. With this H/D ratio, a ratio F.sub.2 /F.sub.1 between the largest cross-sectional area F.sub.1 at the bottom of the crown and the smallest cross-sectional area F.sub.2 at the top of the crown of about 4/5 would be appropriate in order to produce the required pressure gradients for a cooling tower with D of about 40 m. The average slope angle of the tapered crown region is also fixed by the ratios H/D and F.sub.2 /F.sub.1.
A particularly simple construction may be provided when the tapered crown region is conical with straight surface lines. This construction can be produced cheaply and simply, for example by a sheet metal construction. The usual concrete construction can, however, also be used. Alternatively, the crown region may have a continuously curved contour or a contour which is composed of straight sections of different slope.
The design of the cooling tower as thus far described serves mainly for the purpose of preventing cold air penetrations or break-ins at relatively low wind velocities and thus the loss in uplift which is connected therewith. This design is more particularly proposed for cooling towers with natural uplift since the flow of air from forced draught towards is generally at a velocity which would prevent such cold air penetration.
Any tapering of the shell causes acceleration, as a result of which losses will be suffered in draft and thus in efficiency. Therefore, the tapering should be as limited as possible in height and angle with respect to the vertical. On the other hand, the tapering must be sufficient for a positive prevention of penetrations of cold air which may lead to efficiency losses of up to 25%. Thus there exists a need for optimum design specifications regarding the tapering of the internal contour of the shell in the crown region. Beyond the general teaching mentioned above, these should permit the realization of the optimum crown layout in any individual case.
The invention also has for an object to provide the cooling tower designer with design rules which will permit him to determine optimum dimensions for the tapered crown region in case of any absolute cooling tower dimensions and operating conditions occurring in practice, with the consequence that cold air penetrations are prevented positively and, at the same time, the losses in draft caused by the tapering are kept small.
As to the mitigation of the harmful influences of a strong side wind (more than approximately 10 m/s), the cooling tower of the present invention includes a wind-deflecting means having upwardly-sloping deflector surfaces in the crown region adjacent the rim of the cooling tower.
This third feature may be provided in association with the cooling tower crown as described above. The combined use of both features is particularly useful in a natural draught cooling tower. A cooling tower embodying both of these features of the present invention can be operated with optimal efficiency in still air or relatively low wind velocities to avoid the cold air penetrations which then tend to occur, and also with a strong side wind to avoid the partial obstruction of the outlet flow which is generally connected therewith.
Under weather conditions in which cold air penetrations play a subordinate part, but in which there is frequently a high side wind, a design with the wind-guiding means but without the tapered region may be sufficient. This wind deflecting design, moreover, may also be used with cooling towers having artificially generated uplift.
A design of the cooling tower with the tapered crown region alone but without the wind deflecting means is to be preferred under weather conditions in which there is only seldom a side wind and certainly not a high one.
Further objects and advantages of the invention will be apparent from the claims and from the following more detailed explanation of the invention with reference to the several embodiments shown in the accompanying diagrammatic drawings.